The d-block elements are commonly known as transition metals or transition elements

The transition metals are also known as the transition elements or the d-block elements. As the name implies, the chemistry of this group is determined by the extent to which the d electron suborbital levels are filled. Chemical similarities and periodicities can be easily seen horizontally across the d-block of the Periodic Table The chemistry is far from simple, however, and there are many exceptions to the orderly filling of the electron shells,

As previously stated, the chemical properties in the Periodic Table are grouped in two ways: vertically, by group, for similar chemical and some physical properties. Horizontally, by row or period, for consistent periodic changes in the chemical and physical properties. For example, the metals in group 11 have similar characteristics of electrical conductivity, luster, crystal structure, ductility, and tensile strength. Characteristic Properties of Transition Metals Transition metals can be said to possess the following characteristics generally not found in the main grouping of the Periodic Table. They can be mostly attributed to incomplete filling of the electron d-levels.

· The formation of compounds whose color is due to d – d electronic transitions.

· The formation of compounds in many oxidation states, due to the relatively low reactivity of unpaired d electrons.

· The formation of many paramagnetic compounds due to the presence of unpairedd electrons. A few compounds of main group elements are also paramagnetic (e.g. nitric oxide, oxygen).

Color in transition-series metal compounds is generally due to the electronic transitions of two principal types of charge transfer transitions. An electron may jump from a predominantly ligand orbital to a predominantly metal orbital, giving rise to a ligand-to-metal charge-transfer (LMCT) transition. These can most easily occur when the metal is in a high oxidation state. For example, the color of chromate, dichromate and permanganate ions is due to LMCT transitions. Another example is that mercuric iodide, HgI2, is red because of a LMCT transition.

A metal-to ligand charge transfer (MLCT) transition will be most likely when the metal is in a low oxidation state and the ligand is an easily reduced d-d transition. An electron jumps from one d-orbital to another. In complexes of the transition metals, the dorbitals do not all have the same energy.

Transition metal compounds are paramagnetic when they have one or more unpaired delectrons. Some compounds are diamagnetic. These include octahedral, low-spin, d6and square-planar d8 complexes. In these cases, crystal field splitting is such that all the electrons are paired up. Ferromagnetism occurs when individual atoms are paramagnetic and the spin vectors are aligned parallel to each other in a crystalline material. Metallic iron and the alloy alnico are examples of ferromagnetic materials involving transition metals. Anti-ferromagnetism is another example of a magnetic property arising from a particular alignment of individual spins in the solid state.

The transition metals and their compounds are known for their homogeneous andheterogeneous catalytic activity. This activity is attributed to their ability to adoptmultiple oxidation states and to form complexes.

An electron may jump from a predominantly ligand orbital to a predominantly metal orbital, giving rise to a ligand-to-metal charge-transfer (LMCT) transition. These can most easily occur when the metal is in a high oxidation state. For example, the color of chromate, dichromate and permanganate ions is due to LMCT transitions. Another example is that mercuric iodide, HgI2, is red because of a LMCT transition.

A metal-to ligand charge transfer (MLCT) transition will be most likely when the metal is in a low oxidation state and the ligand is easily reduced.

An electron jumps from one d-orbital to another. In complexes of the transition metals, the d orbitals do not all have the same energy. The pattern of splitting of the d orbitals can be calculated using crystal field theory. The extent of the splitting depends on the particular metal, its oxidation state and the nature of the ligands.
In centrosymmetric complexes, such as octahedral complexes, d-d transitions are forbidden by the Laporte rule and only occur because of vibronic coupling, in which a molecular vibration occurs together with a d-d transition. Tetrahedral complexes have a somewhat more intense color because mixing d and p orbitals is possible when there is no center of symmetry, so transitions are not pure d-d transitions. The molar absorptivity (ε) of bands caused by d-d transitions are relatively low, roughly in the range 5-500 M−1cm−1 (where M = mol dm−3).Some d-d transitions are spin forbidden. An example occurs in octahedral, high-spin complexes of manganese(II), which has a d5 configuration in which all five electrons have parallel spins. The color of such complexes is much weaker than in complexes with spin-allowed transitions. In fact, many compounds of manganese(II) appear almost colourless. The spectrum of [Mn(H2O)6]2+ shows a maximum molar absorptivity of about 0.04 M−1cm−1 in the visible spectrum.Transition metal compounds are paramagnetic when they have one or more unpaired d electrons. In octahedral complexes with between four and seven d electrons, both high spin and low spin states are possible. Tetrahedral transition metal complexes, such as [FeCl4]2−, are high-spin because the crystal field splitting is small. This means that the energy to be gained by virtue of the electrons being in lower energy orbitals is always less than the energy needed to pair up the spins.
Some compounds are diamagnetic. These include octahedral, low-spin, d6 and square-planar d8 complexes. In these cases, crystal field splitting is such that all the electrons are paired up. Ferromagnetism occurs when individual atoms are paramagnetic and the spin vectors are aligned parallel to each other in a crystalline material. Metallic iron and the alloy alnico are examples of ferromagnetic materials involving transition metals. Anti-ferromagnetism is another example of a magnetic property arising from a particular alignment of individual spins in the solid state.As implied by the name, all transition metals are metals and conductors of electricity. In general transition metals possess a high density and high melting points and boiling points. These properties are due to metallic bonding by delocalized d electrons, leading to cohesion which increases with the number of shared electrons. However, the group 12 metals have much lower melting and boiling points since their full d subshells prevent d–d bonding. In fact, mercury has a melting point of −38.83 °C (−37.89 °F) and is a liquid at room temperature.Many transition metals can be bound to a variety of ligands. In regards to atomic size of transition metals, there is variation Typically, when moving left to right across the table, there is a trend of decreasing atomic radius. However, in the transition metals, moving left to right, there is a trend of increasing atomic radius which levels off and becomes constant. In the transition elements, the number of electrons are increasing but in a particular way. The number of electrons increase going across a period, thus, there is more pull of these electrons towards the nucleus. However, with the d−electrons, there is some added electron-electronrepulsion. For example, in chromium, there is a promotion of one of the 4s electrons to half fill the 3d sublevel, the electron-electron repulsions are less and the atomic size is smaller. The opposite holds true for the latter part of the row.